Air Laser Gain & Population Inversion

The mechanism of laser-like emission at 391 nm from nitrogen gas irradiated by a strong 800 nm pump laser and a weak 400 nm seed laser is mysterious! The branch ratio of photo-ionization from neutral N2 to N2+ charge state will never result in population inversion between molecular charge state 5 and state 1 in the top left figure. This indicates that no population will produced by photo-ionization. However, we find that the 800nm pump laser does more than ionization! It will simultaneously and coherently drive transitions among all molecular charge states, and generate molecular coherence. We term this mechanism as post-ionization population redistribution mechanism (PPRM). With PPRM, the population inversion significantly shows up and leads to gain at 391nm (see bottom right figure). The molecular coherence is further investigated by two-seed Ramsey interferometry. This is Yu-Hung's second work and published in Opt. Lett. 45, 6587 (2020).